Such monitoring devices are known per se, e.g. from Swiss Pat. No. 518,231. Basically, they operate by biasing a feeler such as a movable thread guide against the stretched filament which normally prevents the feeler from actuating an alarm switch designed to alert the operator to a rupture of the filament and/or to arrest the associated textile machine automatically. This mode of operation makes it necessary to keep the filament under a certain tension so as to prevent a malfunction in the event of a temporary slackening of the thread.
If the feeler has a relatively large moment of inertia, short-term fluctuations in thread tension will not give rise to an untimely alarm condition. With high-speed machinery, such a slow-moving feeler cannot respond quickly enough to the final unwinding of the thread from its supply spool to signal the need for a replacement before the end of the thread has disappeared into the machine. Insertion of a new thread, however, is more easily accomplished if the trailing end of the old one is still within reach.